Three standing marble columns at Pozzuoli posed a geological puzzle: why did a broad band of marine-boring holes run through stone that stood well above the sea? The columns belonged to a Roman monument, yet shell-boring animals had lived inside them. Understanding their marks required explaining changes in the relationship between land and water.
The central mystery is explained. Marine molluscs bored into the submerged marble, and movements of the ground subsequently changed the columns’ height relative to the sea. INGV’s Osservatorio Vesuviano identifies these remains as a historical record of bradyseism—uplift and subsidence in the Campi Flegrei area. That explanation does not mean that every stage of the monument’s history or every nineteenth-century reconstruction is equally secure.
A market misnamed as a temple
The site is commonly called the Temple of Serapis or Serapeum, but the archaeological park identifies it as the Roman city’s food market, or macellum. The traditional temple name followed the discovery of a statue of Serapis. A familiar name had encouraged an interpretation of the building that later archaeology revised.
Lyell used that familiar name in Principles of Geology. In the 1854 edition examined here, he discusses competing antiquarian interpretations before turning to the columns as evidence of physical change. His architectural account must be read as part of that historical inquiry, not substituted for the modern identification of the complex.
The standing columns made the question unusually compelling. Their continued presence allowed observers to compare marks on an ancient structure with the surrounding deposits and the modern sea. The object of investigation was not merely a report that water had once been higher; physical traces remained available for examination.
Exposure of the ruins
Lyell describes the upper portions of the columns protruding through accumulated material before excavations exposed the building in the middle of the eighteenth century. His account places removal of the surrounding soil in 1750. Beneath it lay a paved complex, fallen columns and architectural remains.
Modern park information also describes excavation under Charles of Bourbon after attention was drawn to the visible columns. The recovery of the monument made its submerged history a subject for antiquarians, naturalists and geologists. Discovery and interpretation were different stages: excavation revealed the problem without automatically explaining it.
The marks could not be understood solely from the columns’ impressive appearance. Their distribution, the condition of unmarked surfaces and the material around the bases were all part of the evidence.
Lyell’s measurements and the boring shells
Lyell describes three monolithic marble shafts about forty feet high. Their lower surfaces were relatively uninjured up to about twelve feet above the pedestals. Above that lay a perforated zone roughly nine feet high. His measurements belong to the monument as he recorded it, not a statement of its present-day level.
The cavities were attributed to marine boring bivalves called Lithodomus in his text. Many still contained shells, despite removal by visitors. He describes narrow external openings leading into larger cavities and notes that the size of the borings grew with the animals that made them.
That pattern suggested prolonged immersion, not momentary splashing by a storm. The animals had required time to occupy and enlarge their cavities. At the same time, the absence of borings lower down required explanation: in Lyell’s reconstruction, sediment and building debris had protected the buried lower parts while the exposed middle sections stood in seawater.
He also examined fallen marble columns, including borings on fractured surfaces and attached marine organisms. This extended the evidence beyond a single line on one upright shaft. The ruined structure and its deposits together provided a sequence to interpret.
The observation in 1828
Lyell explicitly records visiting the site in 1828. At that time he placed the uneven platform about a foot below high-water level, with seawater reaching it through the intervening ground. The upper perforations, by contrast, stood far above the contemporary water level.
The combination implied that the standing columns had experienced a period of deep immersion and subsequent emergence. A building erected for use on land could not begin as a functioning market beneath the sea. The interpretation therefore required more than one stage in the relative positions of water, pavement and columns.
Excavation in 1828 also exposed a lower mosaic pavement beneath the visible marble floor. Lyell treated the differing levels as evidence that changes and alterations predated the later monument’s final abandonment. The exact historical reconstruction of those building phases remains separate from the physical identification of marine borings.
Babbage and the surrounding deposits
Lyell credited Charles Babbage’s close examination of deposits and encrustations on the walls and floor. In the reconstruction he discusses, seawater first mixed with water from a local spring, leaving calcareous material that included marine organisms. Other layers included volcanic material, fresh-water carbonate and rubble.
Those observations encouraged a gradual, staged account rather than a single catastrophic plunge. Some proposed links between particular layers and volcanic events remained inferential. Lyell expressly acknowledged that the timing and formation of parts of the accumulated material could not be established with certainty.
He also looked beyond the building. Marine deposits and remains of human structures along the coast offered independent evidence of changed ground levels. The columns mattered because they complemented that wider landscape record, rather than standing alone as an inexplicable architectural anomaly.
Returning to the same place
The investigation did not stop at ancient traces. Lyell recounts Antonio Niccolini’s experience of drawing at the site in 1807, when the pavement ordinarily remained dry in calm weather. On returning to supervise work in 1822, Niccolini found seawater covering it at high tide and needed stones to stand on.
Niccolini’s subsequent observations continued to 1838. Lyell reported his conclusion that the ground was sinking over that period. In a later enquiry, Lyell asked the mineralogist Arcangelo Scacchi to examine the site, and received a report of observations made in the summer of 1852.
Scacchi noted the unequal way in which rising water reached the pedestals, consistent with differences in the pillars’ inclination. He also reported that the downward movement had ceased or become very slight. These returns to an enduring monument helped turn an ancient puzzle into a record that could be compared through time.
What the modern explanation covers
INGV identifies the holes on the standing columns as products of marine molluscs and uses their position and dating in reconstructing historical changes of level. Modern monitoring measures ground movement directly. Together, these lines of evidence explain the core observation: marine borings above the modern sea do not require an unknown civilisation, anomalous stonework or a supernatural flood.
The outcome Closed (Explained) applies to that physical phenomenon. It does not endorse every dimension, architectural identification or proposed event correlation in Lyell’s nineteenth-century account. Nor is this historical case an assessment of the area’s current volcanic activity or a forecast.
Access and map reference
Current access information is inconsistent. The archaeological park’s monument page says temporarily closed, while the named Macellum operator advertises ticketed visits. Interior access is therefore not marked as verified in this case. Confirm the current arrangement with the responsible operator before travelling; a map point does not grant entry.
Approximate map area: 40.8261, 14.1204, rounded from the monument operator’s published map coordinates for the Macellum. The point identifies the archaeological complex, not an individual borehole, a sea-level benchmark or unrestricted access inside the ruins.
Read Charles Lyell’s Principles of Geology, 1854 edition, chapter XXIX, especially pages 509–516, for the measurements and historical observations. The Osservatorio Vesuviano explanation of bradyseism supplies the modern geological identification. The archaeological park’s Macellum record supplies the revised building identification and closure notice; the operator’s website supplies its conflicting visitor listing and map reference. Sources and Wellcome’s CC BY 4.0 image licence were checked on 13 November 2026.